Nondestructive inspection of a high-temperature structure is required to guarantee its safety. However, there are no useful sensors for high-temperature structures. Some of them cannot work at temperatures over 50°C.Another concern is that it is too expensive to use. Therefore, a sensing system, which can transmit and receive an ultrasonic wave and travel a long distance using a long waveguide, has been studied. This means that an ultrasonic sensor could be driven at atmospheric temperature. We could finally confirm that a guided ultrasonic wave generated by a trial electromagnetic acoustic transducer (EMAT) with a thin Ni sheet on the surface of the pipe can travel more than 10m using a thin bar with a 2mm diameter as the waveguide. However, we had difficulty in receiving a reflected ultrasonic wave from the bottom surface of a test specimen. We tried to improve the trial inspection system using an ultrasonic horn. Finally, an experiment in which the temperature of a test block was heated to about 500°C has been done and the reflected ultrasonic wave from the bottom surface of it has been successfully detected using a long waveguide and the wave horn. Finally, we tried to transmit and receive a guided wave in a pipe using the developed system. It was determined that an additional one turn-shaped or wave-shaped waveguide attached to the surface of the pipe in the circumferential direction is useful.